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Logging and Tracing

Logging

Modern applications use logging a lot for debugging and diagnosing a running production service. Usually logs are harvested, indexed and stored in a separate service for further investigation.

The userver framework addresses modern logging requirements and provides multiple facilities for efficient work with logs, including Changing the log level at runtime.

Below are the intruductions to main developer logging facilities.

Log level

Macros are used for logging:

LOG_DEBUG() << "Some debug info, not logged by default in production";
LOG_INFO() << "This is informational message";
LOG_WARNING() << "Something strange happened";
LOG_ERROR() << "This is unbelievable, fix me, please!";

The right part of the expression is calculated only if the logging level is less or equal to the macro log level. I.e., the right part of the expression LOG_DEBUG ()<< ... is calculated only in the case of the DEBUG or TRACE logging level.

Sometimes it is useful to set the logging level of a given log entry dynamically:

logging::Level level = flag ? logging::Level::kDebug : logging::Level::kInfo;
LOG(level) << "some text";

Filter to the log level

Not all logs get into the log file, but only those that are not lower than the logger's log level. The logger log level is set in static config (see components::Logging).

The log level can be changed in the static config for a particular handle. In this case, the log level of the logger is changed only for the request handling task of the handle and for all the subtasks:

yaml
components_manager:
components:
handler-ping:
log-level: WARNING

Dynamic change of the logging level

The logging level that was set in the static config of the components::Logging component for the entire service can be changed on the fly. See Changing the log level at runtime for more info.

Limit log length of the requests and responses

For per-handle limiting of the request body or response data logging you can use the request_body_size_log_limit and response_data_size_log_limit static options of the handler (see server::handlers::HandlerBase). Or you could override the server::handlers::HttpHandlerBase::GetRequestBodyForLogging and server::handlers::HttpHandlerBase::GetResponseDataForLogging functions.

Limiting the log frequency

If some line of code generates too many logs and a small number of them is enough, then LOG_* should be replaced with LOG_LIMITED_*. For example, LOG(lvl) should be replaced with LOG_LIMITED(lvl); LOG_DEBUG() should be replaced with LOG_LIMITED_DEBUG(), etc.

In this case, the log message is written only if the message index is a power of two. The counter is reset every second.

Typical recommended places to use limited logging:

  • resource unavailability

    Examples: insufficient memory; another service is not responding.

    Explanation: in case of a problem such errors are usually reported in thousands per second; their messages do not differ.

  • incorrect use of a component or service

    Examples: negative numbers are passed to your library in a method that accepts only positive ones; the requested functionality is not supported by a third-party microservice.

    Explanation: Such errors should be caught at the testing stage. However, if a miracle suddenly happened and someone did not test the service, then the same errors are be reported in thousands per second

Nuances:

  • Each thread has separate counters, so in practice there may be a little more logs
  • If the template function logs via LOG_LIMITED_X, then each specialization of the function template has a separate counter
  • If the same function with loging via LOG_LIMITED_X is called in different places, then all its calls use the same counter

Tags

If you want to add tags to as single log record, then you can create an object of type logging::LogExtra, add the necessary tags to it and output the LogExtra object to the log:

logging::LogExtra log_extra;
log_extra.Extend("key", "value");
LOG_INFO() << log_extra << "message";

If the same fields must be added to each log in a code block, it is recommended to use tracing::Span, which implicitly adds tags to the log.

Stacktrace

Sometimes it is useful to write a full stacktrace to the log. Typical use case is for logging a "never should happen happened" situation. Use logging::LogExtra::Stacktrace() for such cases:

LOG_ERROR() << "Deadlock in ABC identified"

Important: getting a text representation of a stacktrace is an expensive operation. In addition, the stacktrace itself increases the log record size several times. Therefore, you do not need to use a stack trace for all errors. Use it only when it is 100% useful for diagnostics and other diagnostic tools are ineffective.

Additional loggers

You can create any number of additional loggers. All you need is to add them to your logging.loggers section of the static config (see components::Logging for an example)

After that, you can get a logger in the code like this:

auto& logging_component =
context.FindComponent<::components::Logging>();
my_logger = logging_component.GetLogger("my_logger_name");

To write to such a logger, it is convenient to use LOG_XXX_TO, for example:

LOG_INFO_TO(my_logger) << "Look, I am a new logger!";

Note: do not forget to configure the logrotate for your new log file!

Tracing

The userver implements a request tracing mechanism that is compatible with the opentelemetry standard.

It allows you to save dependencies between tasks, between requests through several services, thereby building a trace of requests and interactions. It can be used to identify slow query stages, bottlenecks, sequential queries, etc.

tracing::Span

When processing a request, you can create a tracking::Span object that measures the execution time of the current code block (technically, the time between its constructor and destructor) and stores the resulting time in the log:

Example log tracing::Span span("cache_invalidate"):

tskv timestamp=2018-12-04T14:00:35.303132 timezone=+03:00 level=INFO module=~Impl ( userver/src/tracing/span.cpp:76 ) task_id=140572868354752 coro_id=140579682340544 text= trace_id=672a081c8004409ca79d5cc05cb5e580 span_id=12ff00c63bcc46599741bab62506881c parent_id=7a7c1c6999094d2a8e3d22bc6ecf5d70 stopwatch_name=cache_invalidate start_timestamp=1543921235.301035 total_time=2.08675 span_ref_type=child stopwatch_units=ms

Log record example for some POST /v1/upload handle:

tskv timestamp=2020-08-13T15:30:52.507493 level=INFO module=~Impl ( userver/core/src/tracing/span.cpp:139 ) task_id=7F110B765400 thread_id=0x00007F115BDEE700 text= stopwatch_name=http/handler-v1_upload-post total_time=36.393694 span_ref_type=child stopwatch_units=ms start_timestamp=1597321852.471086 meta_type=/v1/upload _type=response method=POST body={"status":"ok"} uri=/v1/upload?provider_id=driver-metrics http_handle_request_time=36.277501 http_serialize_response_data_time=0.003394 tags_cache_mapping_time=0.018781 find_service_time=21.702876 http_parse_request_data_time=0.053233 http_check_auth_time=0.029809 http_check_ratelimit_time=0.000118 entities_cache_mapping_time=0.01037 register_request_time=0.819509 log_to_yt_time=0.047565 save_request_result_time=1.523389 upload_queries_time=5.179371 commit_time=4.11817 link=48e0029fc25e460880529b9d300967df parent_link=b1377a1b20384fe292fd77cb96b30121 source_service=driver-metrics entity_type=udid merge_policy=append provider_name=driver-metrics tags_count_append=3 meta_code=200 trace_id=2f6bf12265934260876a236c373b37dc span_id=8f828566189db0d0 parent_id=fdae1985431a6a57

tracing::Span can only be created on stack. Currently, the ability to create tracing::Span as a member of a class whose objects can be passed between tasks is not supported.

Tags

In addition to trace_id, span_id, parent_id and other tags specific to opentracing, the tracing::Span class can store arbitrary custom tags. To do this, Span implicitly contains LogExtra. You can add tags like this:

tracing::Span span("big block");
span.AddTag("tag", "simple tag that can be changed in subspan");
span.AddTagFrozen("frozen",
"it is not possible to change this tag value in subspan");
span.AddNonInheritableTag("local", "this tag is not visible in subspans");

Unlike simple LogExtra, tags from Span are automatically logged when using LOG_XXX(). If you create a Span, and you already have a Span, then LogExtra is copied from the old one to the new one (except for the tags added via AddNonInheritableTag).

Built-in tag semantics

  • TraceId propagates both to sub-spans within a single service, and from client to server
  • Link propagates within a single service, but not from client to server. A new link is generated for the "root" request handling task
  • SpanId identifies a specific span. It does not propagate
  • For "root" request handling spans, there are additionally:
    • ParentSpanId, which is the inner-most SpanId of the client
    • ParentLink, which is the Link of the client

Span hierarchy and logging

All logs in the current task are implicitly linked to the current Span for the user, and tags of this Span are added to them (trace_id, span_id, etc.). All Span in the current task are implicitly stacked, and if there are several similar Span, the last created One (i.e., located at the top of the Span stack of the current task) will be used for logging.

tracing::Span span("big block");
span.AddTag("city", "moscow");
LOG_INFO() << "User " << user << " logged in"; // logs "city" tag
{
tracing::Span span_local("small block");
span_local.AddTag("request_id", 12345);
LOG_INFO() << "Making request"; // logs "city", "request_id" tags
}
LOG_INFO() << "After request"; // logs "city", no "request_id"

If you want to get the current Span (for example, you want to write something to LogExtra, but do not want to create an additional Span), then you can use the following approach:

Creating a Span

A Span is automatically created for each request, handled by the server::handlers::HttpHandlerBase inherited handles. trace_id, parent_id and link are automatically populated for the request headers (if any).

Cache handlers do not have a parent Span, so a Span without a parent is automatically created for them, with a new trace_id for each update. The situation is similar for all utils::PeriodicTask.

When creating a new task via utils::Async, a new Span is created and linked with the current Span of the current task.

DB drivers and the components::HttpClient automatically create a Span for each request to trace them.

Linking service requests

The HTTP client sends the current link/span_id/trace_id values in each request to the server, they do not need to be specified.

When the HTTP server handles the request, it extracts data from the request headers and puts them in the Span.

Names of the headers:

X-YaRequestId
X-YaSpanId
X-YaTraceId

Selectively disabling Span logging

Using the server dynamic config USERVER_NO_LOG_SPANS, you can set names and prefixes of Span names that do not need to be logged. If the span is not logged, then the ScopeTime of this span and any custom tags attached to the span via the methods of the Add*Tag*() are not put into the logs.

For example, this is how you can disable logging of all Span for MongoDB (that is, all Span with stopwatch_name starting with mongo) and Span with stopwatch_name=test:

{
"names": [
"test"
],
"prefixes": [
"mongo"
]
}